1. Field of the Invention
The present invention relates to charge carriers in a semiconductor material and, more particularly, to a method of inducing charge carriers to move through the semiconductor material.
2. Description of the Related Art
Semiconductor devices often utilize an electric field to induce the movement of charge carriers through a semiconductor material from one location to another location. For example, an n+/pxe2x88x92 photodiode is a semiconductor device that utilizes an electric field to collect photo-generated electrons.
FIG. 1 shows a cross-sectional diagram that illustrates a prior art n+/pxe2x88x92 photodiode 100. As shown in FIG. 1, photodiode 100 includes a pxe2x88x92 substrate 110 and an n+ region 112 that is formed in substrate 110. When n+ region 112 is formed in substrate 110, a depletion region 114 is formed that separates substrate 110 from n+ region 112.
In operation, photodiode 100 is first reset by placing a positive potential on n+ region 112 with respect to pxe2x88x92 substrate 110. The potential difference across the n+/pxe2x88x92 junction reverse biases the junction, increasing the size of depletion region 114 and the magnitude of the electric field across the junction.
Once reset, light energy, in the form of photons, is collected by photodiode 100 which forms a number of electron-hole pairs. The electrons from the electron-hole pairs that are formed in depletion region 114 move under the influence of the electric field towards n+ region 112, where each additional electron collected by n+ region 112 reduces the positive potential that was placed on n+ region 112 during reset. On the other hand, the holes formed in depletion region 114 move under the influence of the electric field towards pxe2x88x92 substrate 110.
In addition, the electrons, which are from the electron-hole pairs that are formed in pxe2x88x92 substrate 110 within a diffusion length of depletion region 114, diffuse to depletion region 114 and are swept to n+ region 112 under the influence of the electric field. Further, the electrons that are formed in n+ region 112 remain in n+ region 112.
After photodiode 100 has collected light energy for a period of time, known as the integration period, sense circuitry associated with the photodiode detects the change in potential on n+ region 112. As noted above, the electrons collected by n+ region 112 reduce the magnitude of the positive potential that was originally placed on n+ region 112. Once the change in positive potential has been sensed, photodiode 100 is reset and the process is repeated.
One measure of photodiode 100 is the efficiency with which photodiode 100 can collect the photo-generated electrons. Not all of the electrons from the electron-hole pairs are collected by n+ region 112. Instead, a number of electrons recombine with holes. When an electron recombines with a hole, the photo information associated with the electron is lost.
One limitation of photodiode 100 is that when photodiode 100 is reset, the space charge distribution is unequal. FIG. 2 shows a perspective view that illustrates the n+ region 200 of a prior art n+/pxe2x88x92 photodiode following reset. As shown in FIG. 2, n+ region 200 is a square-shaped area that has a contact region 210.
During reset, a positive voltage is applied to contact region 210 for a predetermined period of time. When the positive voltage is removed, a number of zones of decreasing positive charge, such as zones Z1-Z5, result. When the electrons are then collected during the integration period, an unequal space charge distribution results.
The change in potential that occurs during the integration period can be detected by electrically connecting the potential on n+ region 112 to the gate of a source-follower transistor. An electrical connection to n+ region 112 is typically made by forming a contact on the surface of n+ region 112. However, as shown in FIG. 2, the potential sensed by the contact depends on the zone Z1-Z5 the contact is located in.
The present invention provides a method of inducing charge carriers to move through a semiconductor material to a collection region in the semiconductor material. The method utilizes a conductive trace that is formed over and insulated from the semiconductor material.
In accordance with the present invention, the method includes the step of making a sawtooth current flow through the conductive trace. The sawtooth current induces charge carriers to move through the semiconductor material to the collection region. The sawtooth current has a plurality of periods.
Each sawtooth period has a first edge and a second edge. The second edge has a steeper slope than the first edge, and induces charge carriers to move through the semiconductor material. The first edge, on the other hand, causes substantially no charge carriers to move through the semiconductor material.